A pulse-density modulation (PDM) microphone is a type of digital microphone that is used in many common devices, including mobile phones. PDM microphones use a digital signal (ones and zeros), referred to as a PDM signal, to represent an analog signal. In PDM signals, a density of a particular value of the digital signal (e.g., a value of one) corresponds to the amplitude of the analog signal. In other words, the peaks of the analog signal correspond to a higher density of ones in the PDM signal, while the troughs of the analog signal correspond to a lower density of ones (or a higher density of zeros). Thus, a flat analog input at the maximum amplitude level (e.g., Vdd) corresponds to a stream of ones, a flat analog input at the midpoint (e.g., 0 volts) corresponds to an equal number of alternating zeros and ones, and a flat analog input at the minimum level (e.g., −Vdd) corresponds to a stream of zeros, as described below.
Typically, PDM microphones only produce valid data once a threshold amount of analog input has been sampled. Thus, PDM microphones produce no valid output during a startup period that begins when power and a clock signal are provided to the microphone and ends when enough samples of the input have been taken. As noted, the output from a PDM microphone is either one or zero, which may be interpreted by downstream signal processing circuitry as one and negative one, respectively. Consequently, the startup period can cause problems in the signal processing circuitry because, as there is no valid output, the signal from the microphone during the startup period constitutes a stream of zeros. As noted, the zeros will be interpreted by the signal processing circuitry as negative one, which corresponds to a DC signal with the maximum negative amplitude, which can produce undesired effects, such as an audible click when the output is reproduced as sound.